The present application relates to an organic electroluminescent device (so-called “organic EL device”) and a display device, and specifically to an organic electroluminescent device equipped with an electron transport layer containing a nitrogen-containing heterocycle derivative and a display device.
Organic electroluminescent devices (so-called organic EL devices) which make use of electroluminescence (hereinafter referred to as “EL”) of organic materials are each provided between an anode and a cathode with an organic layer formed of an organic hole transport layer and an organic light-emitting layer stacked one over the other, and are attracting interests as light-emitting devices enabling a high-brightness light emission by a low-voltage direct current drive.
FIG. 12 is a cross-sectional view showing one configuration example of such an organic electroluminescent device. An organic electroluminescent device 51 shown in this figure is arranged on a transparent substrate 52 made, for example, of glass or the like, and is constructed of an anode 53 arranged on the substrate 52, an organic layer 54 arranged on the anode 53, and a cathode 55 arranged on the organic layer 54. The organic layer 54 has a configuration that a hole injection layer 54a, a hole transport layer 54b. a light-emitting layer 54c and an electron transport layer 54d are stacked successively in this order front the side of the anode 53. In this organic electroluminescent device 51, electrons injected from the cathode 55 and holes injected from the anode 53 recombine with each other in the light-emitting layer 54c, and light produced upon this recombination is outputted via the anode 53 or cathode 55. It is to be noted that organic electroluminescent devices also include those of a configuration that a cathode, an organic layer and an anode are stacked successively in this order from the side of a substrate.
Still higher efficiency and longer life are demanded in recent years for such organic electroluminescent devices. For example, 8-hydroxyquinoline aluminum (Alq3) has conventionally been used as the electron transport layer 54d. As Alq3 is low in electron mobility, phenanthroline derivatives (see, for example, Applied Physics Letter (U.S.A.), 76(2). 197-199, Jan. 10, 2000 (hereinafter referred to as Non-patent document 1)) and silole derivatives (see, for example, Applied Physics Letter (U.S.A.), 80(2), 189-191 Jan. 14, 2002 (hereinafter referred to as Non-patent document 2)) have been reported as materials having higher electron mobility than Alq3. The use of these electron transport materials has a merit in that, because the injection of electrons is intensified and the region of recombination between electrons and holes is concentrated on the side of a hole injection electrode (the anode 53), the recombination probability is improved, the luminescence efficiency is raised, and a low-voltage drive is feasible. For the movement of the region of recombination between electrons and holes to the side of the anode 53, on the other hand, more electrons reach the hole transport layer 54b. A triphenylamine derivative commonly employed as the hole transport layer 54b, therefore, becomes very unstable and deteriorates when it accepts electrons. As a consequence, the light-emitting life of the electroluminescent device is shortened.
As attempts to control the carrier balance throughout a device, examples of organic electroluminescent devices each having a layer of high hole transport ability between a light-emitting layer and an electron transport layer have been disclosed (see, for example, the pamphlet of PCT International Publication No. WO 2004/077886 A (hereinafter referred to as Patent document 1) and JP-A-2006-66890 (hereinafter referred to as Patent document 2)).
To prevent short-circuiting between the anode and the cathode to reduce the occurrence of light emission failure, the organic layer is required to have a certain thickness or greater. It is a common practice to choose a hole transport material, which shows high mobility, and to deposit it thick (see, for example, Patent document 1 and JP-A-2005-101008 (hereinafter referred to as Patent document 3)).
As described in Patent documents 1 and 2, the organic electroluminescent device having the layer of high hole transport ability between the light-emitting layer and the electron transport layer is, however, accompanied by a problem in that the electron transport ability is lowered and due an increased drive voltage and insufficient carriers, the current efficiency is deteriorated.
When a material equipped with electron mobility high enough to enable high-efficiency light emission is used as the electron transport layer 54d as described in Non-patent documents 1 and 2, the configuration that the hole transport layer 54a is deposited thick as described in Patent documents 1 and 3 intensifies the supply of electrons owing to the large thickness despite a limitation to the transport of holes, and therefore, involves problems that the earner balance is deteriorated and the life is very shortened.